A sudden often toxic proliferation of marine phytoplankton, notably dinomastigotes, that colours the sea red, brown, or yellowish due to the high concentration of the organisms’ photosynthetic accessory pigments. Some dinomastigotes, such as Gonyaulax, produce potent toxins, which may kill fish and invertebrates outright or accumulate in the food chain, posing a hazard to humans eating shellfish and other seafood. These phytoplankton blooms may be related to nutrient-rich inputs from the land, or upwelling oceanic waters, and are initiated by the activation of cystlike forms lying on the seabed. See also algal bloom.